Luanti
Voxel game-creation platform with easy modding and game creation
...This modular design enables rich customization and community-driven content: users can install new games, mods, and texture packs via a built-in content browser that draws from a central repository (ContentDB). Luanti supports both single-player and multiplayer via servers; players can host their own or join publicly listed ones, with server-side mods being applied automatically.